Typically, in the production of hybrid plants, two breeding lines as parents are crossed with one another, the descendants of which generate, in part, a strongly increased yield relative to the parent lines, due to the known heterosis effect. The breeding lines may be obtained via multiple selfing steps, which, however, takes multiple generations and therefore is connected with an enormous time cost. Modern plant breeding already increasingly transitioned many years ago to generating the breeding lines via haploid induction and the subsequent chromosome duplication in a much shorter amount of time. A technical requirement for this is a functioning haploid induction system, which also simultaneously promises a sufficient efficiency, in order to be economically usable.
For example, for maize (Zea mays), a maternal in vivo induction system is known in which the plants to be induced are pollinated with pollen of the inductor. Up to 10% of the descendants that are thereby generated then contain only the simple (haploid) chromosome set of the seed parent. A few such inductors are presently available for maize hybrid breeding. However, these are all to be ascribed to the single line “Stock 6,” described by Coe, 1959. One example of such a known inductor is the RWS (Rober et al., 2005) line. In the past, multiple QTL studies for the identification of the inductor-relevant loci were conducted on these lines. A main-QTL at chromosome 1 (bin 1.04) in maize was already identified in 1997 by Deimling et al. More precise mapping was performed by Barret et al. 2008 in the range between 66.96 MB and 68.11 MB on chromosome 1, by Prigge et al. 2012 in the range between 62.9 MB and 70.8 MB, and following this by Dong et al. 2013 in the range between 68.18 MB and 68.43 MB which, according to public annotation, contains three genes. All position information refers to the reference genome of B73, Version AGPv02. The functionality of the locus appears to have already been demonstrated on its own by Dong et al. 2014 by achieving an induction rate of 5%. However, an incorrect fine mapping cannot be excluded, since no unambiguous delimitation of the QTL is possible due to the lack of information of flanking markers in the recurrent parent.
Furthermore, WO 2012/030893 discloses an inductor-relevant locus on chromosome 1 in maize that, however, differs markedly from the preceding locus and is localized in more detail at the telomere. There is no overlap in the genome regions considered.
Overall, the molecular and development-specific mechanisms, which the in vivo haploid induction in maize lines which resulted from “Stock 6,” are largely unknown today. For example, it is conceivable that a fertilization occurs, but it subsequently leads to a chromosome elimination which then allows haploid descendants to emerge. For example, such a mechanism has been described by Ravi & Chan (2010) in a system with the histone protein CenH3. On the other hand, however, the fertilization may also fail, and the development of the haploid egg cells occurs in the triploid endosperm. Without the understanding of the underlying maternal in vivo haploid induction suitability of an inductor genotype derived from “Stock 6” and the knowledge about the responsible genes, a targeted improvement of this maize inductor genotype or the transfer of the induction suitability to non-inductor genotypes, or the targeted mediation of the in vivo haploid induction capability in maize non-inductors, is practically impossible.
Furthermore, for some cultivated plants, no efficiently (and, therefore, economically) applicable system for the production of haploid and double-haploid plants is known at all—for example, for sorghum, rye, or sunflower.
There is also a need for the provision of genetic elements such as genes or regulatory elements that are usable in transgenic and/or non-transgenic approaches, in order to enable haploid development, or an improved efficiency in haploid development, via in vivo induction.